It has generally been recognized that an individual's early history influences his responses to stress during adult years. Experimental evidence has demonstrated that shocking infant rats increases the rate of development of the 24-hour adrenocortical rhythm and reduces adrenocortical reactivity at adulthood. The proposed research is directed to three interrelated questions: 1) Do the changes in adrenocortical function that result from early experience reflect changes in pituitary function or are they limited to the adrenocortical system? Growth hormone and prolactin responses to environmental stimulation as a function of early experience will be studied. 2) Which neurotransmitter mediates the effects of early experience on adrenocortical reactivity at adulthood? Neurotransmitters believed to have a role in neuroendocrine regulation will be pharmacologically altered in infant rats. The effects of early experience on adrenal function at adulthood will be studied in these animals. 3) Does early experience influence the rate of development of the 24-hour rhythm of CNS monoamines? CNS monoamines will be localized by immunohistochemical methods and quantified by radioimmunoassay. The 24-hour rhythms in post weaning animals will be determined initially. The age at which the rhythms are expressed and the effects of early experience on the rate of development of the 24-hour rhythms will then be investigated.